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Discussion Starter · #1 ·
I am reading 12 volts on each side of the fuel injector's harness. This means that the voltage coming from the positive side of the batery, through the fuel injection relay, and out to the dropping relay, and then out to the injectors is remaining at 12 volts, and the 12 volts on the other side of the harness is coming from the ECU. I've checked the fuel injection relay, the dropping resistors and everything else that can be checked. Could someone please explain to me why the dropping resistor is not dropping voltage even though it has the correct Ohms? And is the ECU supposed to give 12 volts to all of the injectors at the same time? I mean, even if I was getting a drop in voltage from the dropping resistors to one side of the injector harness to create a potiential difference across the injectors coil, with all of the injectors getting 12 volts as the same time, how are the injectors supposed to cycle? Any comments are welcome.

Thanks,
Max
 

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I don't know where exactly your problem lies, but only the cold-start injector should be getting 12 volts. The other six run on only 3 volts. If they're getting 12, they may be ruined.
 

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Discussion Starter · #3 ·
> I don't know where exactly your problem
> lies, but only the cold-start injector
> should be getting 12 volts. The other six
> run on only 3 volts. If they're getting 12,
> they may be ruined.

I understand. If the injector was getting 12 volts to one side and the other side was grounded, then it probablly would be, but since both sides have 12 volts to them, it's just like no voltage at all. You have helped me to think of checking the cold start injector to see what the voltage is to it. I don't know why I didn't check it first. I guess it's because the book told me to test the ECU by using a test light at the #1 and #4 injectors. When the test light didn't flash, I was told the ECU was bad. So I changed it, but I still don't have the problem fixed. The real problem is that I can't be sure what has happend to the car since I just got it and the guy that had it before me could have done any number of things to it before I got it. I wish someone could tell me something like the ECU pulses impulses of 3 volts to each of the injectors during the intake stroke, and the drop down relay reduces voltage by X# of volts to create a type of ground. As it is, I can't determin what's wrong, because I don't know how these voltages should be, or their relation to one another. Thank you for letting me know that the injectors run on 3 volts. Can anyone reccomend a good book on the subject?

Thanks,
Max
 

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Discussion Starter · #4 ·
> I don't know where exactly your problem
> lies, but only the cold-start injector
> should be getting 12 volts. The other six
> run on only 3 volts. If they're getting 12,
> they may be ruined.

I understand. If the injector was getting 12 volts to one side and the other side was grounded, then it probably would ruin the injectors, but since both sides have 12 volts to them, it's just like no voltage at all. You have helped me to think of checking the cold start injector to see what the voltage is to it. I don't know why I didn't check it first. I guess it's because the book told me to test the ECU by using a test light at the #1 and #4 injectors. When the test light didn't flash, I was told the ECU was bad. So I changed it, but I still don't have the problem fixed. The real problem is that I can't be sure what has happend to the car since I just got it and the guy that had it before me could have done any number of things to it. I wish someone could tell me something like the ECU pulses 3 volts to each of the injectors during the intake stroke, and the drop down relay reduces voltage by X# of volts to create a type of ground. As it is, I can't determine what's wrong, because I don't know what these voltages should be, or their relation to one another. Thank you for letting me know that the injectors run on 3 volts. Can anyone recommend a good book on the subject?

Thanks,
Max
 

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Discussion Starter · #5 ·
Check Cranking RPM

i believe your problem is actually your starting system. If your engine is cranking over slow the car will not start because the injectors are not fired. this is just a normal function of the car. you will get 12 volts out of the ecu in this case. as for getting 12 volts from the positive side of the injector connector, this is only because you're checking the voltage in series. if you checked in parallel with the car running, you'd find a different reading.

casper
 

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Discussion Starter · #6 ·
> I am reading 12 volts on each side of the
> fuel injector's harness. This means that the
> voltage coming from the positive side of the
> batery, through the fuel injection relay,
> and out to the dropping relay, and then out
> to the injectors is remaining at 12 volts,
> and the 12 volts on the other side of the
> harness is coming from the ECU. I've checked
> the fuel injection relay, the dropping
> resistors and everything else that can be
> checked. Could someone please explain to me
> why the dropping resistor is not dropping
> voltage even though it has the correct Ohms?
> And is the ECU supposed to give 12 volts to
> all of the injectors at the same time? I
> mean, even if I was getting a drop in
> voltage from the dropping resistors to one
> side of the injector harness to create a
> potiential difference across the injectors
> coil, with all of the injectors getting 12
> volts as the same time, how are the
> injectors supposed to cycle? Any comments
> are welcome.

> Thanks,
> Max

 Sounds like your ECU is not powering up. The reason you’re seeing 12v on both of the injector wires is because the ECU is not doing anything to complete the circuit. Normally ECU would ground one side of the injector, This completes the circuit and the injectors will drop about 3V while the dropping resistor takes the rest of the 12V. The EFI relay actually has two contacts one goes to the injectors and the other to the ECU. It's the Red wire at the EFI relay that should go to the ECU.
IF you have 12v at the relay, then Unplug ECU connectors and look for 12v there. (Pin 27).
Pass that, check ECU ground wire (pins 19,20,24) with an ohmmeter. or simply measure a known live 12v line to these ground points.
 good luck
tom
 

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Discussion Starter · #7 ·
>  Sounds like your ECU is not
> powering up. The reason you’re seeing 12v on
> both of the injector wires is because the
> ECU is not doing anything to complete the
> circuit. Normally ECU would ground one side
> of the injector, This completes the circuit
> and the injectors will drop about 3V while
> the dropping resistor takes the rest of the
> 12V. The EFI relay actually has two contacts
> one goes to the injectors and the other to
> the ECU. It's the Red wire at the EFI relay
> that should go to the ECU.
> IF you have 12v at the relay, then Unplug
> ECU connectors and look for 12v there. (Pin
> 27).
> Pass that, check ECU ground wire (pins
> 19,20,24) with an ohmmeter. or simply
> measure a known live 12v line to these
> ground points.
>  good luck
> tom
Since you got this car from someone else, did you check the power wires for the fuel injection coming off the battery? There are two one for the plus side and one for the minus, on the 76 it wasn't grounded it went dirtctly back to the minus on the battery. I thought of this since you said you are getting 12 volts from either side of the injector plug. If it is not that possibly a melted wire in the harness causing a short as someone had mentioned.

ddt
 

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Discussion Starter · #8 ·
> ? Sounds like your ECU is not
> powering up. The reason you’re seeing 12v on
> both of the injector wires is because the
> ECU is not doing anything to complete the
> circuit. Normally ECU would ground one side
> of the injector, This completes the circuit
> and the injectors will drop about 3V while
> the dropping resistor takes the rest of the
> 12V. The EFI relay actually has two contacts
> one goes to the injectors and the other to
> the ECU. It's the Red wire at the EFI relay
> that should go to the ECU.
> IF you have 12v at the relay, then Unplug
> ECU connectors and look for 12v there. (Pin
> 27).
> Pass that, check ECU ground wire (pins
> 19,20,24) with an ohmmeter. or simply
> measure a known live 12v line to these
> ground points.
> ? good luck
> tom

Tom, this is a very good post, thank you :eek:) After doing as you suggested, I found out that the wiring harness and the ECU do not match up :eek:( This is really strange since there is only one kind of ECU made for the 8/75 280Z. I have asked the local dealer's service dept. to fax me a diagram of the pin location on the ECU and the wiring harness. I think the guy that owned the car before me rewired the harness to work with a different ECU. Here is what the harness looks like:
___________________________

- - - - - - o o - - o o o o - - o
- - - - - - o - - - - - - - - - - -
___________________________
( - ) = active ( o ) = inactive

The ECU plugs into this ^

If anybody recognizes this pin setup (maybe a newer ECU?) please let me know.

Thanks,
Max
 

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Discussion Starter · #9 ·
Re: Check Cranking RPM

> i believe your problem is actually your
> starting system. If your engine is cranking
> over slow the car will not start because the
> injectors are not fired. this is just a
> normal function of the car. you will get 12
> volts out of the ecu in this case. as for
> getting 12 volts from the positive side of
> the injector connector, this is only because
> you're checking the voltage in series. if
> you checked in parallel with the car
> running, you'd find a different reading.

> casper

Thankyou Casper for the help. I found the problem to be that the ECU and the wiring harness do not match up. I attribute this to the former owner of the car. I think he rewired the harness to work with a different ECU then the one sold for that year of 8/75. There is only one kind made for that year. I've asked a service dept locally to fax me diagrams of the pin locations on the ECU and the harness for that year car. Further on in this thread, I've posted a diagram of what the harness looks like now. The ECU lacks a few pins on one row, and has an extra one on the other. I'll post the diagrams I get from the dealer. If anyone knows off hand this kind of information, please feel free to post a diagram.

Thanks,
Max
 
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